When plunge milling along an edge, or along the walls of a pre-existing pocket or cavity of a workpiece, a milling cutter is subjected to unbalanced radial cutting forces which tend to deflect the milling cutter away from the workpiece. This phenomenon is particularly problematic during deep plunging in which the milling cutter has a long shank, or a long extension shank.
Long shanks tend to be somewhat elastic so that during deep plunging the unbalanced radial cutting forces cause the shank to bend. The deeper the plunging, the greater the bending of the shank. As a consequence, the milling cutter will not plunge in a true axially downward direction, and a resulting milled portion of a surface of the workpiece will be slanted. This is a disadvantage if a true vertical surface is required.